1. Technical Field
The present invention relates to electrostatically actuated Micro-Electro-Mechanical System (MEMS) devices and, more specifically, to optical cross-connects with electrostatically actuated MEMS devices.
2. Art Background
Electrostatically actuated MEMS devices have been proposed for a variety of applications. In one application of such devices, movable micro-machined mirrors are used as a switching element to direct an input optical signal to a desired output. The movement of the micro-machined mirrors is accomplished by electrostatic actuation. One example of a simple electrostatically actuated fiber optic switch is described in Chen, Richard T., et. al., xe2x80x9cA Low Voltage Micromachined Optical Switch by Stress-Induced Bending,xe2x80x9d IEEE (1999). The electrostatically actuated fiber optic switches described in Chen et al. have individual hinged mirrors attached to the end of a polycrystalline silicon (polysilicon) cantilever beam. The cantilever beam is coated with a stressed layer of chromium and gold on its top surface. During operation, a voltage is applied to the cantilever beam, attracting it to the grounded substrate. The mirror is moved in and out of the path of light, redirecting the light to a given, preset output.
One of the disadvantages of the switch described in Chen et al. is that it can accommodate only a limited number of input and output signals. The switch described by Chen et al. has two input fibers and two output fibers. Because of the limited number of inputs and outputs that can be handled by the switch described in Chen et al., that switch is referred to as a low-density switch.
One of the reasons for the low density of a switch architecture that uses the switch elements described in Chen et al. is that the number of switching elements in such architecture is proportional to the square of the number of input/output ports. Thus, a switch with a large number of ports is prohibitively large in size. Also, the light path length and corresponding insertion loss becomes prohibitively large for a large switch that employs such an architecture.
A higher density MEMS optical crossconnect is described in Neilson, David T., et al., xe2x80x9cFully Provisioned 112xc3x97112 Micro-Mechanical Optical Crossconnect With 35.8 Tb/s Demonstrated Capacity,xe2x80x9d Optical Fiber Communication Conference (Mar. 8, 2000). In the crossconnect described in Neilson et al. a 16xc3x9716 array of mirrors is formed on a substrate. The mirrors are raised above the substrate surface by a hinged supporting structure. The hinged supporting structure is attached to the substrate. An electrostatic force moves the mirrors. Supplying an electrical potential to electrodes disposed under the mirrors generates the electrostatic force. In this arrangement, mirrors are tilted to a desired degree to direct the light incident thereon to a desired output in the array of outputs. For a fully provisioned cross connect, only one mirror per input and one mirror per output are required (i.e. the number of ports is N and the number of mirrors is 2N). Thus, in this architecture, the number of mirrors scales with N, not N2 (as in the architecture that uses the Chen et al. elements).
In the crossconnect device described in Neilson et al., an individual mirror element is affixed to a movable supporting structure (i.e. a gimbal) via torsional elements such as springs. The gimbal is coupled to a frame, also via torsional elements. Two torsional elements couple the mirror to the gimbal and the two mirror torsional elements are positioned on opposing sides of the mirror element and define and axis for mirror rotation. Similarly, two torsional elements couple the gimbal to the frame and the two gimbal torsional elements are positioned on opposing sides of the gimbal and define an axis for gimbal rotation. The mirror""s axis of rotation is orthogonal to the gimbal""s axis of rotation. In their relaxed state, these torsional elements keep the movable mirror and gimbal in a plane parallel to the plane of the substrate surface.
Electrodes are positioned directly under the mirror and gimbal. The electrodes are configured to be capable of rotating the mirror element or gimbal in either direction about its axis. The mirror element or gimbal rotates in response to the electrostatic attractive force between the mirror element or gimbal and the fixed electrodes. In an equilibrium position at a given angle of the mirror (zero degrees is the angle in its relaxed, non-tilted state), the attractive force is balanced by the restoring force of the torsional elements. The degree of rotation depends upon the amount of voltage applied to the electrodes. Thus, controlling the amount of voltage applied to the electrode controls the angle of tilt.
The cross-connect described in Neilson et al. is configured so that any of 112 inputs can be connected to any of 112 outputs. In order to provide this number of interconnections, the interconnect (i.e. the mirror array) must be able to direct the input signal to the desired output port. Controlling the tilt angle of the mirror in order to direct an input signal to the desired output port is of great importance. Consequently, the mirror must be tilted with precision. As previously noted, the equilibrium position of the mirror (the electrostatic force between the electrodes and the mirror is balanced by the restoring force of the torsional elements) defines the tilt angle of the mirror. Thus, mechanisms for accurately and precisely controlling the tilt of the mirror are desired.
The present invention is directed to an electrostatically actuated MEMS device. The MEMS actuator device has an actuated element (e.g. an optical element such as a mirror). The actuated element is attached to a supporting structure via torsional elements that define an axis of rotation for the optical element. Typically, two torsional elements affixed to opposing sides of the optical element are provided for this purpose. The supporting structure is supported on a substrate.
In certain embodiments, the supporting structure is moveably attached to a supporting substrate. One example of a movable supporting structure is a gimbal ring. The gimballed configuration provides the actuated element with a second axis of rotation and, consequently, a greater number of mirror positions. The substrate surface underlying the actuated element/support structure has fixed electrodes formed thereon. The combination of electrodes and the actuated element/supporting structure form the electrostatic actuator. The actuated element/supporting structure moves in response to a difference in electrical potential between it and the underlying electrode.
The electrode is configured to generate an electrostatic force between the actuated element and the underlying electrode. The electrostatic force causes the actuated element to rotate about the axis defined by the torsional elements. In one embodiment, a pair of electrodes is provided to effect rotation of the actuated element in both a clockwise and a counter clockwise direction.
The electrode has three components. The first component is the electrode that causes rotation about the axis by providing an electrostatic attractive force between the actuated element and the electrode. The second component is a neutral electrode. As used herein, a neutral electrode is an electrode that is neutral with respect to the actuated element. That is, the neutral electrode is at the same voltage or potential as the actuated element. The third component is configured to compensate for the nonlinear nature of the electrostatic force that causes the actuated element to rotate.
The electrostatic force is nonlinear because, for a given applied voltage, the force increases as the actuated element rotates toward the electrode. At some fixed applied voltage (i.e. a voltage larger than the voltage required to move the optical element) and corresponding degree of rotation (measured as the angle of tilt of the optical element from the planar state), electrostatic force increases at a faster rate than the restoring force of the torsional elements. At this point, the degree of tilt is no longer controllable. Thus, the actuated element is only controllably rotated to some finite angle, after which the rotation becomes uncontrolled.
The third component of the electrode compensates for this nonlinear relationship between the electrostatic force and the restoring force of the torsional elements. Thus, the third component of the electrode extends the range of angles through which the actuated element is controllably rotated (compared to an optical element rotated using a one or a two component electrode).
The configuration of the three component electrode of the present invention is described in terms of its placement in relation to a tilting area defined by the actuated element. For purposes of the present invention, the tilting area of the actuated element is the surface area of the actuated element as projected onto the surface underlying the actuated element. For a given element configuration, the tilting area changes as a function of tilt angle. Typically, tilting area is larger when the actuated element is approximately parallel to the underlying surface (i.e. the tilt angle is about zero degrees) and smaller as the actuated element tilts toward the underlying surface (i.e. the tilt angle gets larger).
For purposes of the present invention, an electrode component is inside the tilting area if at least some portion of that component underlies the actuated element throughout the entire range of tilt. Conversely, an electrode component is outside the tilting area if the entire electrode component lies outside the tilting area through at least some portion of the range of tilt. Consequently, the first and second electrode components are within the tilting area because at least a portion of both the first and second components underlie the optical element throughout the entire range of tilt. The third electrode component is outside the tilting area because, through at least some portion of the range of tilt, the third electrode component is completely outside the tilting area of the optical element. It is advantageous if the third component is completely outside the tilting area of the optical element throughout the entire range of tilt.
The position of the electrode component relative to the tilting area of the actuated element is significant because the position of the electrode component defines the location of the electrostatic field generated by the electrode component. Specifically, the third component of the electrode increases the electrostatic force for a given voltage (compared the force/voltage relationship for an electrode without the third component) when the electrostatic field generated by the third element is higher below the actuated element than above the actuated element. In the context of the present invention, the angles in the range at which the electrostatic field is predominantly below the actuated element are referred to as the small angles of tilt. The third component decreases the electrostatic force for a given voltage (again compared to the force/voltage relationship for an electrode without the third component) when the electrostatic field generated by the third component is higher above the actuated element than below the actuated element. Thus, the bottom side of the actuated element is shielded from at least a portion of the electrostatic field generated by the third component under these conditions.
In the context of the present invention, the angles in the range of angles at which the bottom of the actuated element is shielded from the electrostatic field and the top of the mirror is exposed to the electrostatic field are referred to as the large angles of tilt. For purposes of the present invention, the tilting angle range is zero degrees (defined as the tilt angle of the mirror in its unactuated state) through the range of angles at which rotation is controllable (i.e. the maximum angle of tilt). Thus, the range of angles in which rotation is controlled is extended compared to an actuator having an electrode that is configured to have at least a portion of all electrode components within the tilting area of the optical device.